Various fixes to Python API and scripts

[paulromano]

A summary of the changes in this pull request:

• The merging of OpenMC Python API #331 broke a number of the scripts in utils/ that used the statepoint module. I've updated plot_mesh_tally.py and statepoint_histogram.py to work with the new statepoint module.
• I've deleted statepoint_meshplot.py. This script is essentially an older and more limited version of plot_mesh_tally that doesn't provide any more funtionality.
• I've deleted statepoint_cmp.py. If one is using HDF5, it's easy enough to h5diff two statepoint files.
• Fixed a few errors in the statepoint and tallies modules that I encountered when updated the scripts.
• Moved particle_restart.py to be part of the Python API.

@wbinventor can you review?

[paulromano]

I should also mention that I updated statepoint.py to work with Python 3.

[wbinventor]

Sure, I will take a look at this in the morning.

[wbinventor]

Perhaps we should make Python 3 the standard for Travis? Although I use Python 3 from time-to-time, I still haven't made it the default on my systems, hence issues like this arise. Anyway, just a thought.

[scopatz]

For the record, I use Python 3 exclusively these days. So you have at least one user :). Adding Python 3 to Travis would probably help.

[wbinventor]

There are always the cool kids and a few that are slower than the rest of the pack in every class.

[scopatz]

haha <insert rant about arch linux here>

[paulromano]

I'm inclined to leave Travis using 2.7 for the time being. Most (all except Arch?) distributions use 2 as default still. @scopatz do you have any thoughts about good ways to test both 2 and 3?

[scopatz]

If you add both 2.7 and 3.4 to the Python list in the travis.yml file, it will test both languages in parallel. This is what pyne and cyclus do. See https://github.com/pyne/pyne/blob/develop/.travis.yml#L2 Should be a one or two line change,

[paulromano]

Didn't realize it was that simple - thanks!

[scopatz]

Yep, not hard and totally worth it :)

[wbinventor]

This PR looks good to me save for my few very minor comments.

[wbinventor]

Everything looks good. Thanks for the fixes @paulromano